High efficiency isolated power supplies often employ active switches that replace diodes and operate as synchronous rectifiers to improve efficiency of the power converter. Such power converters may include such switches the secondary side of a transformer. One approach for controlling the switches on the secondary side is to employ a secondary side controller.
FIG. 1 is a schematic of a power converter 100. The power converter 100 includes a primary side 101 and a secondary side 121 of a transformer 110. The primary side 101 includes a plurality of switches 102, 104. The secondary side 121 includes a plurality of switches 122, 124. The plurality of switches 102, 104 of the primary side 101, and the plurality of switches 122, 124 of the secondary side 121 may collectively be referred to as switches 102, 104, 122, 124. Each of the switches 102, 104, 122, 124 may be configured as a diode and a transistor coupled in parallel. The specific operation of the secondary side controllers 126, 128 will be discussed with reference to the operation waveforms of FIGS. 2A and 2B.
FIG. 2A is an operation waveform 200 of a conventional secondary side controller (e.g., secondary side controllers 126, 128 of FIG. 1). Drain to source voltage (VDS voltage) 210 represents the drain to source voltage of the transistor of the switch (e.g., switches 122, 124) on the secondary side 121. Drain to source current (IDS current) 220 represents the current flowing through the switch on the secondary side 121. Gate drive voltage 230 represents the voltage at the gate of the transistor of the switch driven by the secondary side controller. When the gate drive voltage 230 is low, IDS current 220 flows through the body diode. When the gate drive voltage 230 is high, IDS current 220 flows through the switch active area. The secondary side controllers may include internal control functions, such as “blanking time,” which may be implemented as a signal such as a minimum on time 240 and a minimum off time 245, each of which is described below.
To activate (i.e., “turn on”) the switch, the secondary side controller measures the VDS voltage 210 and compares the VDS voltage 210 with a predetermined threshold (i.e., “on threshold” 201), which often corresponds to the forward biased body diode voltage drop (e.g., −0.5V). To deactivate (i.e., “turn off”) the switch, the secondary side controller measures the VDS voltage 210 and compares the VDS voltage 210 with another predetermined threshold (i.e., “off threshold” 202), which often corresponds to the on-state resistance voltage drop with zero current (e.g., 0V).
At the beginning of a power conversion cycle of the input voltage VIN, the VDS voltage 210 may begin to drop. When the VDS voltage 210 crosses the on threshold 201, the secondary side controller asserts the gate drive voltage 230 in order to fully activate the switch. In resonant circuits, the IDS current 220 tends to rise slowly, which may cause the VDS voltage 210 to rise quickly. In some situations, the VDS voltage 210 may rise quickly to the off threshold 202 shortly after the switch is activated. If the VDS voltage 210 rises above the off threshold 202, the secondary side controller may deactivate the switch at the beginning of the power conversion cycle, which may be undesirable. In order to prevent the secondary side controller from deactivating the switch prematurely, the minimum on time 240 may be set and asserted such that the secondary side controller maintains the gate drive voltage 230 of the switch to be high for a minimum amount of time after the gate drive voltage 230 is asserted. In other words, triggering the minimum on time 240 and asserting the gate drive voltage 230 may be dependent upon each other, such as being triggered at the same time. During minimum on time 240, the comparator function of the secondary side controller may be disabled, and the gate drive voltage 230 remains high even if the VDS voltage 210 crosses the off threshold 202.
When the switch is deactivated (i.e., when the VDS voltage 210 crosses the off threshold 202), there may be a temporary sharp drop in the VDS voltage 210. Such a sharp drop may cause the VDS voltage 210 to drop below the on threshold 201. In order to avoid the switch from being activated at the end of the power conversion cycle, the minimum off time 245 may be set and asserted such that the secondary side controller maintains the gate drive voltage 230 of the switch to be low for a minimum amount of time after the gate drive voltage 230 is deasserted. While the minimum off time 245 is asserted, the comparator function of the secondary side controller may be temporarily disabled, and the gate drive voltage 230 remains low even if the VDS voltage 210 crosses the on threshold 201.
FIG. 2B is an operation waveform 250 of a conventional secondary side controller (e.g., secondary side controllers 126, 128 of FIG. 1) showing a potential failure situation. VDS voltage 210, gate drive voltage 230, and minimum on time 240 represent similar signals as those of FIG. 2A. Similarly, the on threshold 201 and the off threshold 202 may be similar to those of FIG. 2A. It is noted that IDS current 220 is not shown in FIG. 2B for simplicity. There may be situations in which the VDS voltage 210 may decrease relatively slowly over the entire power conversion cycle of the input voltage VIN. Because of the VDS voltage 210 decreasing relatively slowly, the VDS voltage 210 may not reach the on threshold 201 for the first time until the end of the power conversion cycle. If the VDS voltage 210 eventually does cross the on threshold 201, the minimum on time 240 and the gate drive voltage 230 may be asserted. Because the minimum on time 240 in this situation has been asserted toward the end of the power conversion cycle, there is a possibility that the VDS voltage 210 increases to above the off threshold 202 before the minimum on time 240 has expired. Because the conventional secondary side controllers are configured to keep the gate drive voltage 230 asserted during the minimum on time 240, the crossing of the off threshold 202 may not be detected and the gate drive voltage 230 may remain activated at the end of the power conversion cycle when the switch is desired to be turned off. If the primary side voltage polarity has changed during this time, there may be cross-conduction between the primary side and the secondary side of the transformer, which may result in a system failure.